Superplastic medium manganese steel and method of produing the same

ABSTRACT

A superplastic medium manganese steel according to the present invention preferably has a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of aluminum (Al), with the remainder being iron (Fe) and inevitable impurities. In another embodiment, a superplastic medium manganese steel according to the present invention preferably has a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of silicon (Si), with the remainder being iron (Fe) and inevitable impurities.

BACKGROUND 1. Technical Field

The present invention relates to a superplastic medium manganese steeland a method of producing the same. More particularly, the presentinvention relates to a superplastic medium manganese steel which doesnot contain expensive components, such as chromium (Cr), nickel (Ni) orthe like, and which exhibits superplasticity without requiring acomplicated pretreatment process, and also relates to a method ofproducing the same.

2. Description of the Related Art

The global demand for automotive steel sheets is expected to continue togrow from about 80 million tons produced in 2015, and the demand forlightweight vehicles is also increasing due to more stringent fueleconomy regulations in each country. Accordingly, there is an increasingdemand for non-ferrous materials for the purpose of reducing the weightof the vehicle body. However, high formability/high strength steelsheets obtained by improving existing steel materials will occupy morethan 80% of automotive steel sheets in the future due to theirlightweight, processing ease and economic advantages. A ferroussuperplastic steel sheet produced according to the present invention isexpected to satisfy the needs of the present industry due to its lowproduction cost, high formability at high temperatures, and highstrength after forming.

From the viewpoint of improvement in the formability of automotive steelsheets, superplasticity has attracted attention. As used herein, theterm “superplasticity” refers to a phenomenon which is caused by grainboundary sliding (other than plastic deformation, dislocation or slip)when materials with fine grain size are tensile-strained at temperaturesabove half of their melting point so as to exhibit extremely highductility (300%) at very low deformation stress. Namely, at deformationtemperatures at which materials exhibit superplasticity, the materialshave low strength and very high ductility, and thus it is possible toform or process complex shapes even via a small amount of force.

Previous studies on superplastic materials have focused on aluminumalloys and zinc alloys, and studies on steel alloys have also beenconducted.

For superplastic steel alloys, two types of alloys have been mainlyresearched. The first type of alloy includes duplex stainless steelswith ferrite-austenite dual-phase structure, which retain a fine grainsize at high temperatures due to high chromium (Cr) and nickel (Ni)content. The second type of alloy includes high-carbon steels in whichfine carbides act as austenite nucleation sites at room temperature andwhich retain a fine grain size at high temperatures.

Previous studies have been conducted extensively on steel alloycompositions for exhibiting superplasticity and on rolling conditions,annealing conditions, and the like during production processes.Furthermore, it is known that both the two types of steel alloy showexcellent formability corresponding to a maximum elongation of over1000% when deformed at a temperature of about 700 to 1200° C.

However, to exhibit superplasticity, duplex stainless steels should havea high Cr content (23 to 34 wt. %) and a high Ni content (4 to 22 wt.%), and sometimes require a high cold-rolling reduction ratio (about90%). In this case, chromium (Cr) and nickel (Ni) are expensivecomponents that cause an increase in the production cost.

High-carbon steel has a total alloying element content lower than thatof duplex stainless steel, but requires complicated pretreatmentprocesses, such as warm rolling and repeated rolling-annealing. Namely,in the production of conventional ferrous superplastic alloys, there isa great economic loss.

In summary, among conventional ferrous superplastic steels, stainlesssteels have an advantage in that they are treated by a general annealingprocess, and thus do not require complicated pretreatment processes, buthave a disadvantage in that they contain extensive Cr and Ni componentswhich significantly increase the production cost. High-carbon steelshave an advantage in that the production cost is reduced becauseexpensive Cr and Ni components are not used, but have the disadvantageof requiring complicated pretreatment processes.

Accordingly, the present invention is intended to provide a superplasticsteel which combines only the advantages of the above-described steels.Namely, the present invention is intended to provide a superplasticsteel whose production cost is reduced because expensive Cr and Nicomponents are not used and which exhibits superplasticity as a resultof performing a general annealing process instead of a complicatedpretreatment process.

PRIOR ART DOCUMENT

[Patent Document]

(Patent Document 1) Korean Patent No. 1387551 (issued on Apr. 15, 2014).

SUMMARY

The present invention has been conceived to overcome the above-describedproblems, and an object of the present invention is to provide asuperplastic medium manganese steel which exhibits superplasticitywithout containing expensive components, such as chromium (Cr), nickel(Ni) or the like, and a method of producing the same.

Another object of the present invention is to provide a superplasticmedium manganese steel which exhibits superplasticity without acomplicated pretreatment process, and a method of producing the same.

The objects of the present invention are not limited to those mentionedabove, and other objects which are not mentioned herein will be clearlyunderstood by a person skilled in the art from the followingdescription.

A superplastic medium manganese steel according to the present inventionhas a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. %or less (excluding 0 wt. %) of aluminum (Al), with the remainder beingiron (Fe) and inevitable impurities.

A superplastic medium manganese steel according to the present inventionhas a composition containing 4 to 8 wt. % of manganese (Mn) and 3 wt. %or less (excluding 0 wt. %) of silicon (Si), with the remainder beingiron (Fe) and inevitable impurities.

The superplastic medium manganese steel according to the presentinvention may further contain 0.2 wt. % or less (excluding 0 wt. %) ofniobium (Nb).

The superplastic medium manganese steel according to the presentinvention may further contain 0.03 wt. % or less (excluding 0 wt. %) ofboron (B).

The superplastic medium manganese steel according to the presentinvention may further contain 0.2 wt. % or less (excluding 0 wt. %) ofcarbon (C).

The medium manganese steel according to the present invention isannealed in the temperature range of a ferrite-austenite dual-phaseregion to form ferrite and austenite.

In the present invention, the temperature range of the dual-phase regionpreferably ranges from 600 to 900° C.

In the present invention, each of ferrite and austenite formed in thetemperature range of the dual-phase region preferably has an averagegrain diameter of 2 μm or less.

A method of producing a superplastic medium manganese steel according tothe present invention includes the steps of: (S1) melting a mediummanganese steel having the composition according to the presentinvention, and then homogenizing the medium manganese steel; (S2)hot-rolling the homogenized medium manganese steel; (S3) cooling thehot-rolled steel; (S4) cold-rolling the cooled steel; and (S5) annealingthe cold-rolled steel at a predetermined elevated temperature.

In the present invention, the temperature for the homogenizing in step(S1) is preferably 1200° C., and the melting in step (S1) is preferablyperformed at a temperature equal to or higher than the homogenizingtemperature.

In the present invention, the hot-rolling in step (S2) is preferablyperformed at a temperature in the range of 1000 to 1200° C.

In the present invention, step (S3) may be performed by any one coolingmethod selected from among water quenching, oil quenching and aircooling.

In the present invention, the cold-rolling in step (S4) is preferablyperformed at a reduction ratio of 90% or less (excluding 0%), morepreferably 60 to 80%.

In the present invention, the cold rolling in step (S4) may be performedat room temperature.

In the present invention, a dual-phase formed in step (S5) is preferablyferrite and austenite.

In the present invention, the annealing in step (S5) is preferablyperformed in the temperature range of a ferrite-austenite dual-phaseregion, and the temperature range of the ferrite-austenite dual-phaseregion preferably ranges from 600 to 900° C.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 shows the microstructure of a specimen that was obtained bycold-rolling inventive steel 1 at a reduction ratio of 60% and thenmaintaining inventive steel 1 at 850° C. for 5 minutes, followed bywater quenching;

FIG. 2 shows the tensile curves of specimens at various strain rates,which were obtained by maintaining inventive steel 1 at 850° C. for 5minutes;

FIG. 3 shows photographs of specimens that were obtained by performingtensile tests on inventive steel 1 at 850° C. and various strain rates;

FIG. 4 shows the microstructure of a specimen that was obtained byperforming a tensile test on inventive steel 1 under the conditions of850° C. and 1×10⁻³ s⁻¹;

FIGS. 5A to 5E show tensile curves and photographs of specimens thatwere obtained by cold-rolling inventive steel 1 at a reduction rate of80% and then performing tensile tests for inventive steel 1 at varioustemperatures and strain rates;

FIGS. 6A to 6C show tensile curves and photographs of specimens thatwere obtained by cold-rolling inventive steel 2 at a reduction rate of80% and then performing tensile tests for inventive steel 2 at varioustemperatures and strain rates;

FIG. 7 shows tensile curves and photographs of specimens that wereobtained by cold-rolling inventive steel 3 at a reduction rate of 80%and then performing tensile tests on inventive steel 3 at 850° C. andvarious strain rates;

FIG. 8 shows tensile curves and photographs of specimens that wereobtained by cold-rolling inventive steel 4 at a reduction rate of 80%and then performing tensile tests on inventive steel 4 at 850° C. andvarious strain rates; and

FIG. 9 illustrates a method of producing a medium manganese steelaccording to the present invention.

DETAILED DESCRIPTION

Embodiments of the present disclosure will be described in detail belowwith reference to the accompanying drawings so that a person havingordinary knowledge in the art to which the present invention pertainscan easily practice the present invention. As can be understood by aperson having ordinary knowledge in the art to which the presentinvention pertains, the following embodiments may be modified in variousforms without departing from the technical spirit and scope of thepresent invention. Throughout the accompanying drawings, the same orsimilar components are designated by the same or similar referencesymbols as much as possible.

The technical terms used herein are used merely to describe specificembodiments, and are not intended to limit the present invention. Eachsingular expression used herein may include a plural expression unlessclearly defined otherwise.

The term “include” or “comprise” used herein specifies a specificfeature, region, integer, step, operation, element, or component, butdoes not exclude the presence or addition of a different specificfeature, region, integer, step, operation, element, component, or group.

All terms including technical terms and scientific terms used hereinhave the same meanings as commonly understood by those having ordinaryknowledge in the art to which the present invention pertains. Termsdefined in commonly used dictionaries should be interpreted as havingmeanings consistent with relevant art documents and the presentdisclosure, and should not be interpreted in an ideal or overly formalsense unless expressly so defined herein.

The present invention is directed to a method of producing a novelferrous superplastic steel that overcomes the problems of conventionalferrous superplastic steels, and encompasses alloy composition ranges,pretreatment processes, and conditions for the exhibition ofsuperplasticity.

Medium manganese steels according to the present invention may includevarious examples as shown in Table 1 below. However, in the followingdescription, the present invention will be described with a focus onexamples of inventive steels 1 to 4.

TABLE 1 Fe—Mn—Al-based steel Fe—Mn—Si-based steel (inventive steel 1)(inventive steel 2) Fe—Mn—Al—Nb-based steel Fe—Mn—Si—Nb-based steel(inventive steel 3) Fe—Mn—Al—B-based Fe—Mn—Si—B-based steel (inventivesteel 4) Fe—Mn—Al—C-based steel Fe—Mn—Si—C-based steelFe—Mn—Al—Nb—C-based steel Fe—Mn—Si—Nb—C-based steel Fe—Mn—Al—B—C-basedsteel Fe—Mn—Si—B—C-based steel

A superplastic medium manganese steel according to the present inventionmay have a composition containing 4 to 8 wt. % of manganese (Mn) and 3wt. % or less (excluding 0 wt. %) of aluminum (Al), with the remainderbeing iron (Fe) and inevitable impurities. This manganese steelcorresponds to Fe—Mn—Al-based steel.

A superplastic medium manganese steel according to the present inventionmay have a composition containing 4 to 8 wt. % of manganese (Mn) and 3wt. % or less (excluding 0 wt. %) of silicon (Si), with the remainderbeing iron (Fe) and inevitable impurities. This manganese steelcorresponds to Fe—Mn—Si-based steel.

The composition of each of the superplastic medium manganese steelsaccording to the present invention may further contain 0.2 wt. % or less(excluding 0 wt. %) of niobium (Nb). Such steels correspond toFe—Mn—Al—Nb-based steel, and Fe—Mn—Si—Nb-based steels, respectively.

The composition of each of the superplastic medium manganese steelsaccording to the present invention may further contain 0.03 wt. % orless (excluding 0 wt. %) of boron (B). Such steels correspond toFe—Mn—Al—B-based steel, and Fe—Mn—Si—B-based steel, respectively.

The composition of each of the superplastic medium manganese steelsaccording to the present invention may further contain 0.2 wt. % or less(excluding 0 wt. %) of carbon (C). Such steels correspond toFe—Mn—Al—C-based steel, Fe—Mn—Al—Nb—C-based steel, Fe—Mn—Al—B—C-basedsteel, Fe—Mn—Si—C-based steel, Fe—Mn—Si—Nb—C-based steel, andFe—Mn—Si—B—C-based steel, respectively.

The superplastic medium manganese steel according to the presentinvention is annealed in the temperature range of 600 to 900° C., whichis the temperature range of a ferrite-austenite dual-phase region,thereby forming ferrite and austenite.

The present specification proposes: (1) the design of a medium manganesesteel that exhibits superplasticity when deformed at high temperatures;(2) a method of producing the medium manganese steel; and (3) tensileconditions for the medium manganese steel. The present invention will bedescribed in detail below.

(1) Design of Superplastic Medium Manganese Steel

Alloys according to the present invention include various steel alloysincluding Mn, Al, Si, Nb, B, and C with the remainder being iron andinevitable impurities (see Table 1). The reasons why the contents ofalloying elements of the steel compositions as described above arelimited will be described below.

Manganese (Mn): 4 to 8 wt. %

Mn is an essential element of the present invention. Mn is an elementimproving hardenability, suppresses austenite-to-ferrite transformationduring cooling after hot rolling, and mostly forms a martensitestructure. A martensite structure containing Mn, when annealed at hightemperatures for superplastic deformation after cold cooling, has a finestructure of 2 μm or less due to the difference in Mn partitioningbetween austenite and ferrite, unlike conventional superplastic ferrousalloys, thus indicating that it is suitable for exhibitingsuperplasticity.

If the Mn content is less than 4 wt. %, there may be a problem in thatthe hardenability of the steel decreases so that ferrite is producedduring cooling after hot rolling so as to form a ferrite single phase ormartensite-ferrite dual-phase structure at room temperature. The ferriteproduced during cooling is likely to suppress superplastic behavior dueto fast recovery and grain growth during high-temperature deformationafter cold rolling.

In contrast, if the Mn content is more than 8 wt. %, problems may arisein that the material cost and the production cost increase and in thatthe weldability of the steel decreases and a large amount of inclusionMnS is formed. In addition, an excessively high content of Mn can lowerthe ferrite-austenite dual-phase region temperature to cause anaustenite single phase at temperatures above about half of the meltingpoint that exhibits superplasticity, thus causing grain coarseningattributable to rapid grain growth. Therefore, in the present invention,the Mn content preferably ranges from 4 to 8 wt. %.

Aluminum (Al): 3 wt. % or Less (Excluding 0 wt. %)

This limitation is applied to steels containing Al. Like Mn, Al alsopartitions between austenite and ferrite phases at deformationtemperatures, and thus contributes to achieving a fine grain size. Al isknown as a ferrite stabilizer, and increases the ferrite-austenite dualphase region temperature to enable a ferrite-austenite dual-phase to beformed during deformation at superplastic temperatures. Materials havinga dual-phase structure at superplastic temperatures have abundantinterphase boundaries, and the interphase boundaries are effective ininhibiting grain growth during deformation.

In contrast, if the Al content is more than 3 wt. %, there may occurproblems, including increases in the material cost and the productioncost, difficulty in continuous casting, a reduction in weldability, andthe like.

In addition, the addition of a large amount of Al produces ferrite atthe hot-rolling temperature, in which the ferrite is likely to causecoarse grains attributable to fast recovery and grain growth duringhigh-temperature deformation after cold rolling. Therefore, in thepresent invention, the Al content is preferably 3 wt. % or less(excluding 0 wt. %).

Silicon (Si): 3 wt. % or Less (Excluding 0 wt. %)

This limitation is applied to steels containing Si. Like Al, Si is aferrite stabilizer and is known as a strong solid solution strengtheningelement. By virtue of the solid solution strengthening effect, Si isexpected to increase the internal strength of grains at hightemperatures, thereby promoting grain boundary sliding. In addition, Siis known to have an excellent effect of suppressing cementiteprecipitation, and is expected to suppress grain boundary slidinginterference caused by cementite that can be precipitated by carbon (C)at high temperatures.

In contrast, if the Si content is more than 3 wt. %, there may occurproblems, including increases in the material cost and the productioncost, a decrease in cold reduction ratio, a decrease in weldability,etc. Therefore, in the present invention, the Si content is preferably 3wt. % or less (excluding 0 wt. %).

Niobium (Nb): 0.2 wt. % or Less (Excluding 0 wt. %)

This limitation is applied to steels containing Nb. Nb is known as anelement that inhibits the growth of recrystallized grains after coldrolling. The addition of Nb is expected to achieve finer grains to thusform a plurality of grain boundaries, thereby promoting grain boundarysliding.

However, if the Nb content is more than 0.2 wt. %, there may occurproblems, including an increase in the material cost, the precipitationof a second phase, a reduction in recrystallization rate, etc.Therefore, in the present invention, the Nb content is preferably 0.2wt. % or less (excluding 0 wt. %).

Boron (B): 0.03 wt. % or Less (Excluding 0 wt. %)

This limitation is applied to steels containing B. If an excessivelylarge number of vacancies occur at grain boundaries during deformationat high-temperatures, the vacancies can grow so as to initiate andpropagate cracks, thus resulting in low elongation. B is expected tosegregate into grain boundaries at high temperatures to thus increasethe atomic density at the grain boundaries, thereby inhibiting crackgeneration.

However, if the B content is more than 0.03 wt. %, the amount of B thatsegregates to grain boundaries at high temperatures can increase to thusinhibit grain boundary sliding. In addition, it can strain concentrationduring deformation due to boride precipitation at high temperatures,thus resulting in low elongation. Therefore, in the present invention,the B content is preferably 0.03 wt. % or less (excluding 0 wt. %).

Carbon (C): 0.2 wt. % or Less (Excluding 0 wt. %)

This limitation is applied to steels containing C. C is an austenitestabilizer that controls the ferrite-austenite content at hightemperatures. In addition, C is an austenite-strengthening element thatcan strengthen the inside of grains to thus promote grain boundarysliding. However, C is an element that diffuses rapidly between ferriteand austenite, and, in many cases, segregates to grain boundaries athigh temperatures. The amount of C that segregates is maximally 4 timeslarger than the amount of alloying elements, and is particularly largeat grain boundaries.

If the C content is more than 0.2 wt. %, the amount of C that segregatesto grain boundaries at high temperatures can increase to thus inhibitgrain boundary sliding. Furthermore, it can be precipitated as cementiteat temperatures higher than about half of the melting temperature (whichis superplastic temperature) to thus cause stress concentration duringdeformation, thus resulting in low elongation. Meanwhile, a high Ccontent can result in a decrease in weldability. Therefore, in thepresent invention, the C content is preferably 0.2 wt. % or less(excluding 0 wt. %).

Table 2 below shows tensile properties that appear in each type of steelduring deformation at high temperatures. The deformation temperature inTable 2 is defined as the temperature at which the ratio of ferrite toaustenite in each type of steel is 1:1.

TABLE 2 Deformation Composition (wt. %) temperature Strain rateElongation Mn Al (° C.) (s⁻¹) (%) Pretreatment Inventive 6.6 2.3 850 1 ×10⁻¹ 241 1. Cold steel 1 1 × 10⁻² 596 reduction 1 × 10⁻³ 1014 ratio: 60%Comparative 6.7 0.1 645 1 × 10⁻¹ 30 2. 5 minutes steel 1 1 × 10⁻² 78 ofmaintenance 1 × 10⁻³ 233 at deformation Comparative 8.5 0.1 620 1 × 10⁻³137 temperature, steel 2 followed by deformation

Table 3 below summarizes tensile properties that appear during thehigh-temperature deformation of steels produced according to the methodof the present invention.

TABLE 3 Deformation Composition (wt. %) temperature Strain rateElongation Mn Al Si Nb B (° C.) (s⁻¹) (%) Pretreatment Inventive 6.6 2.30 0 0 650 1 × 10⁻¹ 100 1. Cold steel 1 1 × 10⁻² 186 reduction 1 × 10⁻³450 ratio: 80% 700 1 × 10⁻¹ 158 2. 5 minutes 1 × 10⁻² 306 of maintenance1 × 10⁻³ 705 at deformation 800 1 × 10⁻¹ 247 temperature, 1 × 10⁻² 867followed by 1 × 10⁻³ 1196 deformation 850 1 × 10⁻¹ 337 1 × 10⁻² 1113 1 ×10⁻³ 1314 1 × 10⁻⁴ 848 900 1 × 10⁻¹ 382 1 × 10⁻² 962 1 × 10⁻³ 971Inventive 7.02 0 2.04 0 0 600 1 × 10⁻¹ 48 steel 2 1 × 10⁻² 58 1 × 10⁻³387 650 1 × 10⁻¹ 124 1 × 10⁻² 269 1 × 10⁻³ 871 700 1 × 10⁻¹ 204 1 × 10⁻²610 1 × 10⁻³ 1000 Inventive 6.67 2.26 0 0.05 0 850 1 × 10⁻¹ 291 steel 31 × 10⁻² 745 1 × 10⁻³ 1072 Inventive 6.69 2.28 0 0 0.003 850 1 × 10⁻¹278 steel 4 1 × 10⁻² 770 1 × 10⁻³ 1003

(2) Production Method

A method of producing a superplastic medium manganese steel according tothe present invention will be described below. FIG. 9 illustrates amethod of producing a superplastic medium manganese steel according tothe present invention.

As described above, the present invention is directed a method ofproducing a superplastic steel which combines only the advantages of thestainless steel and high-carbon of conventional superplastic ferroussteels. Namely, the present invention is directed to a method ofproducing a superplastic steel whose production cost is reduced becauseexpensive Cr and Ni are not used and which exhibits superplasticity as aresult of performing a general annealing process instead of acomplicated pretreatment process. The present invention is technicallycharacterized in that a medium manganese steel having a compositionaccording to the present invention is produced by a general annealingprocess without requiring a complicated pretreatment process.

The method of producing the medium manganese steel according to thepresent invention includes the steps of: (S1) melting a medium manganesesteel having each of the compositions of various examples as describedabove, and then homogenizing the medium manganese steel; (S2)hot-rolling the homogenized medium manganese steel; (S3) cooling thehot-rolled steel; (S4) cold-rolling the cooled steel; and (S5) annealingthe cold-rolled steel at a predetermined elevated temperature.

In the present invention, the temperature for the homogenizing in step(S1) is preferably 1200° C., and the melting temperature in step (S1) ispreferably equal to or higher than the homogenizing temperature. Thetemperatures corresponding to step (S1) are temperatures that aregenerally used, and the homogenizing temperature in the presentinvention was set at 1200° C. In an example of the medium manganesesteel according to the present invention, an ingot obtained by castingafter melting was homogenized at a temperature of 1200° C. for 12 hours,and hot-rolled at a temperature of about 1000 to 1200° C., which is thetemperature of austenite single phase region. After hot rolling, thesteel was water-quenched or air-cooled in order to prevent ferrite frombeing produced during cooling. The hot-rolled steel mostly has amartensite structure. The structure after hot rolling should be mostlymartensite in order to increase the possibility of achievingsuperplasticity through cold rolling and annealing as proposed in thepresent invention.

In the present invention, the hot-rolling temperature in step (S2)preferably ranges from 1000 to 1200° C. If the hot-rolling temperatureis higher than 1200° C., energy loss can be caused during thehot-rolling process. If the hot-rolling temperature is lower than 1000°C., a ferrite phase can be produced during the hot-rolling process, andthe produced ferrite can grow into coarse grains during subsequentsuperplastic deformation. This can inhibit the superplastic performanceto be achieved by the present invention. For these reasons, thehot-rolling temperature preferably ranges from 1000 to 1200° C. asdescribed above.

In the present invention, step (S3) is performed by any one coolingmethod selected from among water quenching, oil quenching and aircooling. In an example of the present invention, the water quenchingmethod was selected in order to avoid ferrite transformation duringcooling after hot rolling and to obtain a martensite structure. However,when the difference in microstructure after hot rolling between coolingrates was actually investigated, it could be seen that not only thewater quenching method, but also the oil quenching method and the aircooling method, showed no ferrite transformation, and made it possibleto obtain a martensite structure in most cases. The present inventionalso encompasses increasing cooling efficiency by use of a combinationof the water quenching, oil quenching and air cooling methods.Meanwhile, when the fact that superplasticity is achieved by the aircooling method is taken into account, it can be seen that the actualapplicability of the air cooling method to the industry is very high.

In the present invention, step (S4) is preferably performed at areduction ratio of 90% or less (excluding 0%). The medium manganesesteel according to the present invention has athermal martensite afterhot rolling. A structure with fine grains can be formed at thedual-phase temperature after cold rolling by introducing deformationsuch as dislocation into martensite. In addition, as cold reductionratio increases, finer grains can be obtained, and, for this reason, thereduction ratio more preferably ranges from 60 to 80%. In one example,the hot-rolled steel was cold-rolled at each of reduction ratios of 60%and 80% at room temperature.

In the present invention, the cold rolling in step (S4) may be performedat room temperature. Room temperature is generally the temperature atwhich steel sheets are cold-rolled, and a special additional process isnot required for cold rolling at room temperature. For this reason, thecold rolling temperature in the present invention is preferably roomtemperature.

In the present invention, the annealing temperature in step (S5) ispreferably in the temperature range of a ferrite-austenite dual-phaseregion. The medium manganese steel according to the present invention,when annealed, undergoes the reverse transformation of martensitestructure so as to have a ferrite or austenite structure. If theannealing temperature is higher than the dual-phase region temperature,the steel will have an austenite single phase. In contrast, if theannealing temperature is lower than the dual-phase region temperature,the steel will have a ferrite single phase. In the temperature range ofthe dual-phase region, the steel has a ferrite-austenite dual phase, inwhich case grains and interphase boundaries increase.

At lower temperatures in the temperature range of the dual-phase region,the fraction of ferrite is higher. Furthermore, as the temperatureincreases, the fraction of ferrite decreases and the fraction ofaustenite increases.

It is generally known that when grain boundary sliding is activated,superplasticity is promoted. Accordingly, in the present invention, inorder to achieve superplasticity using a plurality of grain boundaries,the annealing temperature was set in the temperature range of thedual-phase region. In one embodiment, the temperature range of thedual-phase region for superplastic deformation may be set to the rangeof 600 to 900° C.

(3) Tensile Conditions

The tensile temperature was set to a temperature in the range of 600 to900° C. by referring to the experimental results of comparative steelsshown in Table 2 above and a temperature of 1773K (1500° C.) which isthe melting point of the alloy. In the given temperature range, thehighest elongation is expected at the point at which the ratio offerrite to austenite is 1:1. The reason for this is that a plurality offerrite-austenite interphase boundaries interferes with grain growthduring deformation. In addition, the reason for this is that a pluralityof interphase boundaries and grain boundaries promote grain boundarysliding.

The steel was maintained for five minutes before deformation afterheating up to deformation temperature so that austenite reversetransformation occurred sufficiently. In this case, microstructures athigh temperatures showed a ferrite-austenite dual-phase structure havinga grain size ranging from about 0.3 to 2 μm. As used herein, the term“grain size” refers to the average grain diameter of ferrite andaustenite grains.

The present invention will be described with reference to theaccompanying drawings below.

FIG. 1 shows the microstructure of a specimen that was obtained afterinventive steel 1 had been cold-rolled at a reduction ratio of 60% andthen maintained at 850° C. for 5 minutes, followed by water quenching.In FIG. 1, α represents ferrite, and α′_(F) represents martensiteproduced by transformation of high-temperature austenite during cooling.In this case, the grain size of ferrite and austenite grains is 2 μm orless. This suggests that the specimen manufactured by the productionmethod proposed in the present invention has a fine grain size.

FIG. 2 shows the tensile curves of inventive steel 1 at various strainrates and 850° C. In this case, the strain rate is strain per second,and was set to a strain rate of 1×10⁻¹ s⁻¹ or less. FIG. 3 shows theappearance of specimens that were obtained after performing tensiletests under the conditions shown in FIG. 2. From FIGS. 2 and 3, it canbe seen that inventive steel 1 exhibits superplasticity under theproduction method of the present invention and the tensile conditions.In addition, as can be seen from the results in FIG. 2, high elongationis expected, particularly at a slow strain rate of 1×10⁻³ s⁻¹ or less.The reason for this is viewed as being that grain boundary slidingoccurs sufficiently due to the slow strain rate.

FIG. 4 shows the microstructure of a specimen that was obtained byperforming a tensile test on inventive steel 1 under the conditions of850° C. and 1×10⁻³ s⁻¹. As can be seen therein, grains show an equiaxedshape similar to that before deformation (FIG. 1), thus demonstratingthat grain boundary sliding occurred actively during tensile deformationat high temperature. For this reason, inventive steel 1 can exhibitsuperplasticity at high temperatures.

FIGS. 5A to 5E show tensile curves and photographs of specimens thatwere obtained by cold-rolling inventive steel at a reduction ratio of80% and then performing tensile tests on inventive steel 1 at varioustemperatures and strain rates. From FIGS. 5A to 5E, it can be seen thatinventive steel 1 exhibits superplasticity under the above-describedproduction process and deformation conditions.

FIGS. 6A to 6C show tensile curves and photographs of specimens thatwere obtained by cold-rolling inventive steel 2 at a reduction ratio of80% and then performing tensile tests on inventive steel 2 at varioustemperatures and strain rates.

FIG. 7 shows tensile curves and photographs of specimens that wereobtained by cold-rolling inventive steel 3 at a reduction ratio of 80%and then performing tensile tests on inventive steel 3 at 850° C. andvarious strain rates.

FIG. 8 shows tensile curves and photographs of specimens that wereobtained by cold-rolling inventive steel 4 at a reduction ratio of 80%and then performing tensile tests on inventive steel 3 at 850° C. andvarious strain rates.

From the above-described tests and data, it can be seen thatsuperplastic medium manganese steels (inventive steels 1 to 4) have beenfinally developed.

The superplastic medium manganese steel according to the presentinvention has a total alloying element content of about 10 wt. % orless, which is lower than half of that of the total alloying element ofconventional duplex stainless steel, thus indicating that it is verycost-effective and is also effective in saving limited naturalresources. Furthermore, the production method according to the presentinvention is simplified to a cold rolling process following hot rolling,which is a conventional process for producing a commercial steel sheet,thus suggesting that it is actually easily applied to the industry.

In addition, the superplastic medium manganese steel according to thepresent invention exhibits an elongation of over 1000% at a temperaturein the range of about 600 to 900° C., and thus shows formabilitycomparable to that of conventional ferrous superplastic alloys.Furthermore, it has the advantage of having high strength at roomtemperature because austenite is transformed into martensite duringcooling after high temperature deformation.

The superplastic medium manganese steel according to the presentinvention is expected to be widely applied to aerospace materialsrequiring high strength and high formability, such as turbine blades,building interior and exterior materials having complex shapes, and carbody steel sheets, such as car hoods, trunks or pillars.

The superplastic medium manganese steel and the method of producing thesame according to the present invention have the following effects.

First, the superplastic medium manganese steel according to the presentinvention has the effect of exhibiting superplasticity withoutcontaining expensive components, such as chromium (Cr), nickel (Ni) orthe like, which have been required in conventional superplasticstainless steel sheets. This also has an additional effect of reducingthe production cost.

Second, the superplastic medium manganese steel according to the presentinvention has the effect of exhibiting superplasticity without acomplicated pretreatment process which has been performed for theproduction of conventional high-carbon superplastic steel sheets.Namely, superplasticity is achieved through the application of generalprocess procedures, whereby the actual applicability of the steel to theindustry is improved and the steel productivity is increased.

The effects of the present invention are not limited to those mentionedabove, and other effects which are not mentioned can be clearlyunderstood by a person skilled in the art from the above detaileddescription.

Although the specific embodiments of the present invention have beendisclosed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible without departing from the scope and spirit of the invention asdisclosed in the accompanying claims.

What is claimed is:
 1. A method of producing a superplastic mediummanganese steel, the method comprising the steps of: (S1) melting eithera medium manganese steel having a composition containing 4 to 8 wt. % ofmanganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of aluminum (Al),with the remainder being iron (Fe) and inevitable impurities, or amedium manganese steel having a composition containing 4 to 8 wt. % ofmanganese (Mn) and 3 wt. % or less (excluding 0 wt. %) of silicon (Si),with the remainder being iron (Fe) and inevitable impurities, and thenhomogenizing the medium manganese steel; (S2) hot-rolling thehomogenized medium manganese steel; (S3) cooling the hot-rolled steel;(S4) cold-rolling the cooled steel; and (S5) annealing the cold-rolledsteel at a predetermined elevated temperature, wherein a microstructureof the superplastic medium manganese steel undergoes a reversetransformation from a martensite single phase structure to aferrite-austenite dual phase structure.
 2. The method of claim 1,wherein a temperature for the homogenizing in step (S1) is 1200° C., andthe melting in step (S1) is performed at a temperature equal to orhigher than the temperature for the homogenizing.
 3. The method of claim1, wherein the hot rolling in step (S2) is performed at a temperature ina range of 1000 to 1200° C.
 4. The method of claim 1, wherein step (S3)is performed by at least one cooling method selected from among waterquenching, oil quenching and air cooling.
 5. The method of claim 1,wherein the cold rolling in step (S4) is performed at a reduction ratioof 90% or less (excluding 0%).
 6. The method of claim 5, wherein thereduction ratio ranges from 60 to 80%.
 7. The method of claim 1, whereinthe cold rolling in step (S4) is performed at room temperature.
 8. Themethod of claim 1, wherein the predetermined elevated temperature rangesfrom 600 to 900° C.
 9. The method of claim 8, wherein each of theferrite and the austenite has an average grain size of 2 μm.